1. Field of the Invention
The present invention relates to an optical information recording medium, and in particular, to an optical information recording medium having a high recording density, such as a Blu-ray Disc in which a dye is used in a recording layer.
2. Description of the Related Art
Optical information recording media such as optical discs have been widely used as information recording media. An example of such an optical information recording medium is a CD Recordable (CD-R) disc produced by sequentially forming a reflective layer and a recording layer on a light-transmissive resin substrate having a thickness of 1.2 mm and a diameter of 120 mm or 80 mm. Recently, a high information recording density has been desired. Consequently, a system that uses a laser beam having a short wavelength and an objective lens having a large numerical aperture (NA) has been studied, and an optical information recording medium such as a DVD Recordable (DVD±R) disc has been realized. In order to increase the tolerance of a tilt angle (tilt) of a disc caused by a decrease in the wavelength and an increase in the NA, the DVD±R disc has a structure in which two light-transmissive resin substrates each having a thickness of 0.6 mm are bonded and a reflective layer and a recording layer are provided between the substrates.
However, recently, in order to record high-resolution graphic data, a higher information recording density has been desired. To meet this requirement, as in a Blu-ray Disc Recordable (BD-R) disc, an optical information recording medium has been proposed in which a reflective layer and a recording layer are provided at the light-incident side of a resin substrate having a thickness of 1.1 mm and a light-transmissive cover layer having a thickness of 0.1 mm is provided thereon.
As described in Japanese Unexamined Patent Application Publication No. 2003-45079, in an optical information recording medium, a reflective layer and a recording layer are sequentially formed on a resin substrate which has a thickness of 1.1 mm and in which a guiding groove (i.e., pregroove) is provided on a surface thereof, and a light-transmissive resin cover layer having a thickness of 0.1 mm is provided thereon. This optical information recording medium has the same diameter and thickness as CD-R and DVD±R discs. As disclosed in Japanese Unexamined Patent Application Publication No. 2003-36562, in order to protect a recording layer, a protective layer made of a light-transmitting inorganic material may be provided between the recording layer and a cover layer. The recording layer of such an optical recording medium is made of an organic substance containing a dye such as an azo dye or a cyanine dye; or an inorganic substance such as Si, Cu, Sb, Te, or Ge. Data is recorded in the recording layer by forming pits by laser irradiation for recording.
However, BD-R discs including a recording medium made of an organic substance are disadvantageous in that the degree of modulation is lower than that of BD-R discs including a recording medium made of an inorganic substance. Since a change in the optical path length of a recording layer made of an organic substance tends to be smaller than that of, for example, a recording layer made of an inorganic substance, the degree of modulation becomes low. Accordingly, one idea for increasing the change in the optical path length is to increase the depth of a guiding groove. However, an increase in the depth of the guiding groove increases the thickness of the recording layer. As a result, the recording layer contains a large amount of dye, and thus the reflectance before recording tends to be decreased. Consequently, the change in the optical path length decreases instead, thereby further decreasing the degree of modulation.